The present invention relates to a method for manufacturing a semiconductor device and, more particularly, to a method for manufacturing a high-breakdown voltage semiconductor device.
In order to improve a breakdown voltage of, e.g., a power transistor, various improvements have been proposed. For example, a structure is proposed wherein an electric field is prevented from being locally increased in a base region, an electric field in the base region is averaged, and an electric field is widened. In such a structure, a guard ring method is normally employed. An RFP (Resistive Field Plate) method, a JTE (Junction Termination Extension) method, and the like have been studied, and have been applied to some mass-production techniques.
The RFP method is described in literature entitled "High-Voltage Large Area Planar Devices" IEEE, Electron Device Letters, EDL-2, September 1981, No. 9, pp. 219-221. The JTE method is described in literature entitled "Junction Termination Extension (JTE), A New Technique For Increasing Avalanche Breakdown Voltage and Controlling Surface Electric Fields In P-N Junctions" IEDM, 77, pp. 423-426.
However, in an element employing the guard ring method, an electric field is concentrated on a curvature portion of a base-collector junction portion of a transistor and a junction curvature portion of a guard ring portion surrounding it. For this reason, in order to obtain a high-breakdown voltage element, the number of guard rings must be increased. However, if the number of guard rings is increased, the effective element area that can be used may be decreased. In the RFP method, a problem of "concentration of an electric field on the base region" as the drawback of the guard ring method is decreased. However, a leakage current level of a device is higher than that in the guard ring method, and a breakdown voltage of only about 70% of an ideal one can be obtained.
In the JTE method, an photoetching process and an impurity diffusion process are repeatedly performed in addition to manufacturing processes of the base region, so that a low-concentration layer is formed on a wide range around the base region, thus forming a base-collector junction portion having a large curvature. In the JTE method, a breakdown voltage 90% or higher of an ideal one can be obtained. However, in order to obtain a junction portion having a large curvature, the photoetching and impurity diffusion processes must be repeatedly performed. For this reason, the manufacturing processes become complex. In addition, breakdown voltages of manufactured semiconductor devices vary due to mask misalignment during the manufacturing processes.